Here are steps you can take to troubleshoot and resolve this issue:

1. Check PAM Configuration:

   • Review the PAM configuration files, typically found in the /etc/pam.d/ directory. Verify if there are any misconfigurations or discrepancies.

2. Check for PAM Module Errors:

   • Look for any error messages related to PAM modules in system logs (e.g., /var/log/auth.log, /var/log/secure). These logs may provide more specific details about what's causing the authentication failure.

3. Verify User and Password:

   • Ensure that the username and password you're using are correct. Double-check for typos or any other mistakes.

4. Check for Account Locks:

   • Verify that the user account (psaadm) is not locked or disabled. Some systems have mechanisms in place to prevent login after a certain number of failed attempts.

5. Check Permissions and Ownership:

   • Verify that the necessary files and directories related to PAM have correct permissions and ownership. Incorrect permissions can cause authentication issues.

6. Restart Plesk Services:

   • Restart the Plesk services to ensure that any temporary issues with authentication or PAM sessions are resolved.

7. Review PAM Modules and Configuration:

   • Check which PAM modules are being used for authentication. Make sure they are correctly configured and that any required dependencies are installed.

8. Check for Disk Space and Inodes:

   • Verify that there is sufficient disk space and available inodes on the system. A lack of resources can lead to authentication failures.

9. Consult Documentation or Support:

   • Refer to the official Plesk documentation or reach out to Plesk support for specific guidance on resolving PAM-related issues.

10. Consider Rollback:

    • If the issue started occurring after a recent change or update, consider rolling back to a previous configuration or version to see if that resolves the problem.

11. Monitor Logs for Further Information:

    • Keep an eye on system logs for any additional error messages or information related to PAM and authentication.

Remember to exercise caution when making changes to system configurations, especially when dealing with authentication mechanisms, as incorrect changes can potentially lock you out of the system. If you're uncertain, consider seeking assistance from a qualified system administrator or support team.